Image replication system

ABSTRACT

The present invention relates, in one aspect to an image replication system comprising a light guide ( 2 ), a light source ( 8 ) arranged to direct emitted light into a first end ( 2   a ) of a light guide ( 2 ), a reflective spatial light modulator ( 6 ) and focussing means ( 16 ). In a preferred embodiment, the light guide ( 2 ) is in the form of an elongate glass rod having first and second parallel polygonal end faces ( 2   a,    2   b ) which are parallel to the long axis of the rod, the rod having a uniform polygonal cross section along its length. The spatial light modulator ( 6 ) is optically coupled to the second end face ( 2   b ) of the light guide ( 2 ) and the focussing means ( 16 ) is arranged to receive light reflected back through the rod from the spatial light modulator ( 6 ), whereby to form multiple images of said spatial light modulator in an image plane ( 18 ).

[0001] In a first aspect, the present invention relates to a system forproviding illumination of substantially uniform intensity at a location,and in a second aspect to an image replication system.

[0002] There are several known ways of generating a holographic image.In one method, light modulated by a series of first spatial lightmodulators (eg. electrically addressed liquid crystal devices) is passedthrough an array of lenses and focussed onto a second spatial lightmodulator, whereby a real image is formed on the surface of the secondspatial light modulator. Coherent light from a laser impinging on thisreal image can be used to produce the holographic image in aconventional manner. Such a system has a number of drawbacks which areaddressed by the present invention. Holographic imaging is only oneapplication in which the systems of the present invention are useful.

[0003] It is an object of the present invention to provide a system forproviding illumination of substantially uniform intensity at a location.It is a further object of the present invention to provide an improvedimage replication system. It is a still further object to provide animproved holographic imaging system.

[0004] According to a first aspect of the present invention, there isprovided an illumination system comprising:

[0005] (i) a light source, and optically coupled thereto,

[0006] (ii) an elongate light guide having a polygonal cross sectionalong its length,

[0007] wherein, in use, non-collimated light derived from the lightsource enters the light guide at its first end and is transmitted to itssecond end, and wherein the intensity of transmitted light at saidsecond end is substantially uniform over the area of said second end.

[0008] According to a second aspect of the present invention, there isprovided an image replication system comprising:

[0009] (i) an elongate light guide having a polygonal cross sectionalong its length,

[0010] (ii) a light source arranged to direct emitted light, in use,into the light guide at its first end,

[0011] (iii) a reflective first spatial light modulator opticallycoupled to a second end of the light guide, and

[0012] (iv) focussing means arranged to receive light reflected throughthe light guide from the first spatial light modulator, said reflectedlight having exited the light guide through its first end, and to formmultiple images derived from said first spatial light modulator in animage plane.

[0013] The light source may be a source of polarised or non-polarisedlight. Preferably, light entering the light guide is non-polarised.Thus, said system may comprise means for converting polarised light tonon-polarised light between the light source and the light guide.

[0014] The light guide may be in the form of a solid glass rod (or rodof other optically transparent material) having first and secondparallel polygonal end faces, said end faces being perpendicular to thelongitudinal axis of the rod. Such a rod, particularly when highlypolished, will act as a light guide by virtue of total internalreflection of light passing through the rod incident with the sides ofthe rod. Use of such a rod is attractive since it can be polished usingnonstandard manufacturing techniques. However, it is desirable to usevery high quality optical glass to minimise wavefront errors due toglass inhomogeneities.

[0015] Preferably, the light guide is of square or rectangular crosssection. In the case of a solid (eg, glass) rod, the edges arepreferably bevelled to produce a minor surface. More preferably thebevel depth is about 1% or less of the distance between adjacent edges(i.e. less than 1% of the width of the rod sides). The minor surface maybe highly polished or diffuse. It will be understood that the beveldepth may vary along the length of the rod and/or the bevel depth of theedges may differ from each other.

[0016] Alternatively, the light guide may be a hollow pipe. The internalsurfaces of the pipe must be suitably reflective or a reflective coatingmust be applied to the internal surfaces. As an example, the hollow pipecan be constructed from discrete mirror elements which are mounted toform the pipe (specifically, four mirrors can be mounted at 90° to eachother to form a square or rectangular sectioned pipe). In an alternativevariation, the pipe may be moulded from one or more separate components(eg. plastic, glass or metal components) which, if a reflective coatingis required, may be coated before or after formation of the pipe in thecase of a pipe moulded from two or more components or after formation ofthe pipe in the case of moulding from a single component.

[0017] In the case of a hollow pipe, transparent end faces arepreferably provided. Such end faces prevent the ingress of dust andother contaminants into the light guide. The provision of end facesallows the interior of the light guide to be sealed from the outsideatmosphere. The interior may be under vacuum or gas filled (eg. drynitrogen) or liquid filled.

[0018] The light source may be a source of collimated or non-collimatedlight. In the former case, a light spreading element such as a diffuseris disposed in the light path between the light source and the lightguide.

[0019] Preferably the first spatial light modulator modulates thepolarisation state of incident light, and is more preferably anelectrically addressable spatial light modulator (EASLM). Mostpreferably, the EASLM comprises a pixelated liquid crystal layer on areflective silicon backing layer, switched pixels causing modulation ofthe polarisation state of incident light, unswitched pixels causing nosuch modulation.

[0020] Alternatively, the first spatial light modulator may be one whichmodulates reflectivity (eg. an array of electrically controlled etalons)or one which modulates angular deflection of incident light (eg. amicromirror array). In the latter case, it will be understood that theangular deflection induced at unwanted pixels must be sufficiently largethat light reflected therefrom is outside the area of interest at theimage plane.

[0021] Since EASLM's are generally rectangular, the light guideconveniently has a rectangular cross section, although it will beappreciated that the cross-section could be any regular or non-regularpolygon.

[0022] In a first series of embodiments, the system comprises apolariser in the light path between the light guide and the focussingmeans, oriented such that only light reflected from switched pixelspasses therethrough to form an image in the image plane. It will beunderstood that such embodiments are designed for use with polarisedlight entering the light guide.

[0023] In a second series of embodiments, a polariser is providedbetween the light guide and the first spatial light modulator, with afractional-wave plate being provided between the polarise and the firstspatial light modulator, wherein the fractional wave plate is chosensuch that only light passing through the polariser and fractional waveplate and incident on a switched pixel, passes back through the waveplate and polariser upon reflection from the spatial light modulator. Itwill be understood that such embodiments are particularly suited for usewith unpolarised light. It will be understood that in this series ofembodiments (and in those embodiments where the EASLM does not modifythe polarisation state of incident light, the light guide may beprovided with a silver, aluminum or other reflective coating.

[0024] In a highly preferred embodiment of the second series, switchedpixels of the EASLM are designed to induce a 90° rotation ofpolarisation state of incident light, and said fractional-wave plate isa quarter-wave plate.

[0025] In practice, the light guide will not be in direct contact withthe object plane of the first spatial light modulator. This separationcan result in “lost pixels” (i.e. pixels which are not imaged properly)at the edges of the first light modulator. In order to overcome thisproblem, the light guide is preferably provided with an additional lightsource around its periphery at an end of the light guide which, in usewill be adjacent the first spatial light modulator. More preferably,said additional light source comprises a plurality of optical fibres, inwhich case a diffusing element is preferably provided between theoptical fibres and the first spatial light modulator. In a particularlypreferred embodiment, a light steering element (eg. a diffractive) isprovided (preferably between the additional light source and thediffusing element when present) to direct light emitted from theadditional light source generally towards the edges of the first spatiallight modulator.

[0026] An image screen may be provided for viewing images formed in theimage plane. Alternatively, a photsensitive material may be provided inthe image plane (eg. photosensitive paper for printing). Preferably,however, a second spatial light modulator is provided in the imageplane. More preferably, the second spatial light modulator is anoptically addressable spatial light modulator and most preferably amodulatable liquid crystal layer on a photoconducting film (eg. aferroelectric liquid crystal light modulator on amorphous silicon).

[0027] Preferably, means are provided to produce an image from lightreflected from the first spatial light modulator in a predeterminedregion of the image plane. Said means may comprise an electro-optical ormechanical shutter (located as closely as possible to the image plane toavoid inter-image cross-talk or loss of pixels at image boundaries).Alternatively, if a second spatial light modulator is in the imageplane, the second spatial light modulator may be selectably sensitisedto incident light.

[0028] As a further alternative, the system may be arranged to producean image only in a predetermined region of the image plane. This may beachieved by the provision of means to introduce an angular bias to lightfrom the light source before it enters the light guide (eg. a scanningmirror) couple to a narrow angle diffuser.

[0029] The illumination system (or image replication system) maycomprise a plurality of light guides. Said light guides may be of thesolid transparent type or hollow. Preferably, the light guides arearranged in an array such that adjacent light guides are mutuallyspaced. Conveniently, channels between adjacent light guides providemeans for delivering light to the first ends of the light guides.

[0030] Preferably, the illumination system comprises means forefficiently delivering light from the light source to the light guide.Said light delivery means preferably comprises a beam splitter betweenthe light source and the light guide and an optical relay between thelight source and the beam splitter. In a preferred arrangement, theoptical relay comprises a pair of mutually spaced lenses with a variableaperture stop therebetween.

[0031] Embodiments of the invention will now be described by way ofexample only, with reference to the accompanying drawings, in which,

[0032]FIG. 1 is a schematic representation of an image replicationsystem in accordance with the present invention,

[0033]FIG. 2 is a schematic representation of part of the system shownin FIG. 1, showing the passage of light along the light guide,

[0034] FIGS. 3 to 5 are schematic representations of part of the systemshown in FIG. 1, showing image formation on the screen,

[0035]FIG. 6 is a schematic representation of an optical relay for usein the embodiment of FIG. 1,

[0036]FIG. 7a is a schematic representation of a different imagereplication system in accordance with the present invention,

[0037]FIGS. 7b and 7 c are detail views of part of the system of FIG.7a,

[0038]FIGS. 8a and 8 bshow mounting arrangements for a bevelled-edgelight guide,

[0039]FIG. 9 is a schematic representation of part of the system of FIG.1, illustrating the “lost pixel” problem.

[0040]FIG. 10 is a schematic representation of part of an imagereplication system according to the present invention adapted toovercome the lost pixel problem.

[0041]FIG. 11 is a schematic representation of part of yet another imagereplication system in accordance with the present invention,

[0042]FIGS. 12 and 13 are schematic representations of light guidearrays forming part of a system in accordance with the presentinvention, and

[0043]FIG. 14 is a schematic representation of part of a system forgenerating a holographic display.

[0044] Referring to FIG. 1, a system in accordance with the presentinvention comprises a light guide 2 made from a solid block of opticalglass having a rectangular cross section and optically flat first andsecond end faces (2 a, 2 b) which are each perpendicular to thelongitudinal axis of the block. The four sides of the block are eachparallel to the longitudinal axis of the block and are also opticallyflat. A beam splitter 4 is optically coupled to the first end face 2 aof the light guide 2 and an electrically addressed spatial lightmodulator (EASLM) 6 is optically coupled to the second end face 2 b ofthe light guide 2. The EASLM (of per se known type) is a pixelatedrectangular liquid crystal display mounted on a reflective siliconlayer. Electrically switched pixels induce 90° rotation in thepolarisation state of light incident thereon. It will be understood thatsince the EASLM 6 is rectangular, it is convenient for the light guide 2to have a rectangular cross section, (in addition, the cross section ofthe light guide 2 matches the size of the object data area on the EASLM6). Although if other non-rectangular spatial light modulators areemployed, the light guide 2 could be constructed with any appropriatepolygonal cross section.

[0045] The system also comprises a laser 8 (in this case an argonlaser), a beam steering prism 10, a light spreading element in the formof a diffuser 12 and a first polariser 14. The (optional) beam steeringprism 10 directs light from the laser 8 towards the beam splitter 4. Theuse of such a prism 10 facilitates a more compact design of system sincethe laser 8 can be arranged parallel to the light guide 2 as shown inFIG. 1. The diffuser 12, which in this case is a holographic diffuser ofan array of small lenses is positioned between the prism 10 and the beamsplitter 4. It will be understood that any other type of diffuser (suchas a spinning diffuser) could be employed. The first polariser 14 ispositioned between the diffuser 12 and the beam splitter 4.

[0046] The system also comprises an imaging lens 16 and an imagingscreen 18, both of which are arranged on the longitudinal axis of thelight guide 2 adjacent the beam splitter 4. The design of the lens 16determines the required length of the light guide 2. A second polariser20 is positioned between the beam splitter 4 and the imaging lens 16 onthe longitudinal axis of the light guide 2.

[0047] In use, polarised light from the laser 8 passes through thesteering prism 10 and towards the diffuser 12. The diffuser 12 causesthe light to become non-collimated with a specific intensitydistribution as a function of angle. The first polariser 4 ensures thatonly polarised light passes into the beam splitter 4. Approximately halfthe light is directed from the beam splitter 4 into the light guide 2 atits first end face 2 a. It will be understood, from the foregoing thatthe intensity of light over the first end face 2 a of the light guide 2is not uniform. As shown in FIG. 2, light incident on the sides of thelight guide 2 is internally reflected, the number of reflections beingdependent upon the angle of light relative to the longitudinal axis ofthe light guide 2. The spatial distribution of light reaching the EASLM6 is substantially uniform (i.e. uniform intensity distribution) andprovides good illumination of the EASLM 6.

[0048] Considering FIG. 3, light incident on the EASLM 6 at relativelysmall angles will be reflected back to the first end face 2 a of thelight guide 2 without reflection on the sides of the light guide 2. Whenthis light is focussed by the imaging lens 16, an image 24 of the EASLM6 will be formed on the screen 18. It will be understood that lightincident on the EASLM 6 is modulated by the EASLM 6 such that switchedpixels represent an “object” 22 formed on the EASLM 6, the image 24formed on the screen therefore being an image of this object 22. Lightincident on unswitched pixels is unmodulated. The second polariser 20 isorientated perpendicularly to the first polariser 14 such thatunmodulated light from unswitched pixels does not reach the screen.Although the EASLM 6 in the above embodiment operates by altering thepolarisation state of light incident on switched pixels, otherembodiments may be envisaged in which switched pixels are reflective andnon-switched pixels are light transmissive or absorbing, such that onlylight incident on switched pixels is reflected towards the screen 18. Insuch embodiments either one or more usually both of the first and secondpolarisers 14,20 may be omitted.

[0049] Considering FIG. 4, light reflected from the EASLM 6 at a certainrange of (wider) angles will be reflected from one of the side faces ofthe light guide 2 before leaving the light guide 2 at its first end face2 a. This results in an additional image 24 a on the screen, the lightappearing to emanate from another (virtual) object 22 a. Due to theaxial symmetry of the illumination, there will be a virtual object 22 bon the opposite side of the longitudinal axis giving rise to a furtherimage 24 b on the screen 18 (FIG. 5). If light rays are twice reflected,five images will result. In general, n reflections will result in 2n+1images. Furthermore, there will be reflections in the perpendicular anddiagonal orientations resulting in multiple images throughout the wholeof the plane of the screen 18, resulting in a total of (2n+1)² images,where n is determined by the refractive index and dimensions of thelight guide 2 and the angular extent of the diffused source. Thus, therefractive index of the block is chosen according to the desired numberof replicated images.

[0050] For optimum illumination resulting in good intensity distributionacross the image plane, it is required that the size of the illuminationpatch on the diffuser be larger than the light guide width to ensurethat the full required numerical aperture fills the first end 2 a(pupil) of the light guide. This results in wastage of a significantportion of the light incident on the diffuser. The fraction of lightusefully coupled into the light guide is approximated by P²/(P+4B tanθ)²), where P is the width of the light guide (for a square crosssection), B is the dimension of the beam splitter and θ is the diffusionangle. For example, where P=10 mm, B=20 mm and θ=10°, then the opticalthroughput efficiency will be approximately 0.503.

[0051]FIG. 6 shows an optical relay (4f arrangement) for deliveringefficient illumination of the required numerical aperture across thelight guide pupil in the embodiment of FIG. 1. The relay comprises apair of mutually spaced lenses 25 with an adjustable aperture stop 26halfway therebetween. A relay stop 27 limits the size of the collimatedillumination patch entering the relay from the diffuser 12.

[0052] The relay aperture top 26 also acts as a field stop for theimaging system, as it limits the angular range of rays entering thefirst end (pupil) 2 a of the light guide 2. Therefore, careful controlof the relay aperture stop 26 (and therefore the imaging field stop)allows a mechanism for direct control of the illumination distributionat the final image plane 18. One advantage is that the inclusion of animaging field stop (aperture stop 26 in the relay) sharply limits thesize of the final image field (i.e. the number of replications),reducing the need for baffling if many light guides are butted together(as may be required for large active tiling systems—see below). A secondadvantage arising from the inclusion of a well defined field limitingstop is that the introduction of a field-apodising element is nowpossible (not shown), such that field dependent attenuation may beapplied to offset image intensity variations, via an amplitudetransmission mask occupying the stop 26.

[0053] A further advantage of the relay arrangement is that stray lightdue to back reflections from the pupil surroundings towards the imageplane 18 (resulting in veiling glare at the image and hence performancedegradation) is reduced to zero.

[0054] The light guide 2 is mounted using square-apertured metal rings(not shown) located at intervals (for example at the Airy points of thelight guide) along its length. The light guide is bonded to the rings bypotting adhesive, the light guide being coated with a light absorbingcoating at those locations so that light incident at those locationsdoes not contribute to an image in use. In other embodiments, whereunpolarised light is used (see below), a reflective (eg. metallic)coating can be applied at the fixing locations. In order to reduce theeffect of light loss on the final image, the area of the coatings iskept to a minimum.

[0055] In the above embodiment, although the intensity of light reachingthe EASLM 6 is uniform, glass inhomogeneity and phase changes imposed byinternal reflections have a deleterious effect on the polarisation ofthe illuminating rays. The polarisation state of a ray emerging from thelight guide 2 at its second end face 2 a will depend on the angulardeviation of the ray from the longitudinal axis of the light guide 2 andthe glass inhomogeneity. One solution to this problem is to place anadditional polariser between the second end face 2 b of the light guide2 and the EASLM 6. Unfortunately, this causes a variation in thebrightness (intensity per unit solid angle) across the angular ray cone,which in turn will vary spatially across the second end face 2 b of thelight guide.

[0056] An alternative solution is to illuminate the EASLM 6 withperfectly unpolarised light, such as from an LED or any othernon-polarised light source (if light of a single wavelength is required,an appropriate filter arrangement can be used). There will be no spatialor temporal correlation between vertically and horizontally polarisedcomponents of the light, and glass inhomogeneities and internalreflections will have no discernible effect on the polarisation state ofthe light. As a result, there will be no spatial/angular dependence ofpolarisation state at the second end face 2 b of the light guide 2.Thus, by including a polariser between the light guide 2 and the EASLM6, the EASLM 6 is illuminated with polarised light having spatiallyuniform intensity and brightness.

[0057] It will be appreciated that when the EASLM 6 is one whichmodulates light by changing its polarisation state (eg. a liquid crystalpanel as described above in which switched pixels act as a half-waveplate and rotate the polarisation of incident light by 90°) lightreflected from switched pixels will not pass back through the polariser.The polarisation state of light incident on non-switched pixels isunchanged upon reflection and so will pass back through the polariser.It will therefore be understood that the image formed on the screen willbe an inverse or negative image of the object. A positive image can beachieved by the inclusion of a quarter-wave plate.

[0058] Referring to FIG. 7a, the image replication system shown isessentially the same as that of FIG. 1 (corresponding elements beingreferred to by the same reference numerals), except that the first andsecond polarisers 14,20 are omitted, and the laser 8 is replaced by asource of unpolarised light 30. A polariser 32 is disposed between thesecond end face 2 b of the light guide 2 and the EASLM 6, with aquarter-wave plate 34 disposed between the EASLM 6 and the polariser 32.The fast axis of the quarter-wave plate 34 is arranged to be at 45° withrespect to the polariser 32.

[0059]FIG. 7b illustrates the effect on the polarisation state of lightexiting the light guide 2 incident on switched pixels 36. Firstly, onlypolarised light (in this case vertically polarised) reaches thequarter-wave plate 34. The vertically polarised light is converted toright-hand circularly polarised light by the quarter-wave plate 34 andis subsequently converted to left-hand circularly polarised light by aswitched pixel 36 of the EASLM 6. When the reflected light is passedback through the quarter-wave plate 34 it is converted back tovertically polarised light (since the light is traversing the wave plate34 in the opposite direction, the fast axis is orientated at −45° to thepolariser 32). The reflected vertically polarised light passes throughthe polariser 32 and back into the light guide 2. As describedpreviously, the light reflected from the switched pixels 36 forms imageson the screen 18.

[0060]FIG. 7c illustrates the effect on polarisation state of lightincident on unswitched pixels 38. As described previously, verticallypolarised light is converted to right hand circularly polarised light bythe quarter-wave plate 34 on its way towards the EASLM 6. There is nochange to the polarisation state upon reflection from the unswitchedpixels 38. Further passage through the quarter-wave plate 34 results inthe light having a horizontal polarisation. The horizontally polarisedlight does not pass through the polariser 32. As result, a positiveimage is formed on the screen 18 from light reflected only from theswitched pixels 36.

[0061] In the above described embodiments, the light guide 2 isdescribed as being of rectangular cross-section (or of cross sectioncorresponding to the EASLM 6). The requirement for precise 90° anglesalong the sides of the light guide (square or rectangular section) doesnot lend itself to easy manufacture. Such edges are easily damaged (eg.chipped) during manufacture resulting in wasted light guides andincreased manufacturing costs. This problem may be overcome by a slightmodification of the above embodiment (not shown) in which the lightguide edges are bevelled at a bevel angle of 45°, resulting in each 90°angled edge being replaced by two 135° angled edges which are less proneto chipping. In other embodiments it will be appreciated that the bevelangel may be different.

[0062] In one variation of the modification, the resultant bevelledsurface is highly polished. Light will internally reflect from theseinternal surfaces in the same way as the main light guide surface,thereby resulting in diagonally shifted images being produced on theimage screen, the distance of each image from the main image beingrelated to the number of internal reflections and the bevel depth.Although these additional “ghost” images have a negative effect on theoverall image quality, the seriousness of the effect is determined bythe relative areas of the bevelled and main surfaces (which determinesthe relative intensities of the desired and ghost images). Thus, with asquare section light guide having sides of about 10 mm, the bevel depthis kept to about 100 μm. This bevel depth is sufficient to mitigate theproblem of chipping but small enough such that the ghost images are soweak so as not to affect the overall image quality to any significantextent.

[0063] In a second variation, the bevelled surfaces are roughened toform a diffusing surface. Although this will not generate ghost images,there will be a general veiling glare across the image region. As withthe polished bevelled surfaces, the detrimental effect is related to therelative areas of the bevelled and main light guide surfaces. At a beveldepth of 100 μm (for a 10 mm square cross-sectioned light guide) theglare is insignificant.

[0064] Referring to FIG. 8a, the bevelled light guide 2 can be mountedin a channelled mounting block 80. The mounting block 80 has slopingsides 80 a whose angle of slope matches the bevel angle of the lightguide edges, and which is machined such that a pair of bevelled edgescan be adhered to the sloping sides 80 a of the mounting block 80, theintermediate main surface of the light guide standing clear from thebase of the channel. If a non-absorbing adhesive 82 is used, thebevelled edges may be provided with a light absorbing coating. In theembodiment shown, the sloping sides 80 a are planar. In an alternativeembodiment, the sloping sides 80 a are provided with castellations sothat the light guide 2 is only in contact with the sloping sides 80 a orthe mounting block 80 at discrete points along its length.

[0065] In a slight variation of the above mounting arrangement, thelight guide can be mounted between an opposed pair of mounting blocks 80which are clamped together as shown in FIG. 8b. In this arrangement, noadhesive is required, although a light absorbing coating on the bevellededges is still desirable.

[0066] In practice, the light guide will not be in contact with theobject plane. This is clearly the case for the embodiment described withreference to FIGS. 7a to 7 c where there is a compensation wave plateand a polariser between the EASLM 6 and the light guide 2. Even in theembodiment described with reference to FIG. 1, the EASLM 6 has a coverglass (not shown) resulting in a small separation between the lightguide and the object plane. Referring to FIG. 9, this separation resultsin “lost pixels” at the edge of the display 6. The number of pixels lostat each edge is given by (s/p)tan θ where s is the separation betweenthe light guide and the object plane (eg. EASLM 6), p is the pixel pitchand θ is the extreme ray angle. θ is dependent on the image beingconsidered, and increases for images further away from the primaryimage. Thus, the lost pixel problem increases for images moving awayfrom the centre as θ increases.

[0067] Referring to FIG. 10, an optical fibre bundle 60 is positionedaround the periphery of the light guide at its second end 2 b, held inplace by cladding 62 (a diffractive 64 and a diffuser 66 are positionedat the light emitting end of the optical fibre bundle 60, their purposebeing described below). The bundle is separated from the light guide 2only by a reflective light blocking film 68 to maintain the internalreflecting properties of the light guide 2 to its second end 2 b and toprevent light leakage between the guide 2 and the optical fibre bundle60. In other embodiments (not shown) separate bundles (or other lightsources) are provided along each side of the light guide 2. To ensureimage continuity, the spacing between the light guide 2 and the bundle60 must be less than the pixel pitch of the object plane (eg. EASLM 6).To recover all lost pixels, the width of the optical fibre bundle 60must be equal to s tan θe, where θe is the maximum ray angle necessaryto produce the image required furthest from the central image.

[0068] It will be understood that a significant portion of the lightemitted by the optical fibres is directed away from the object plane.The diffractive 64 imposes an angular bias to the light (eg. by an angleθe/2. The diffractive 64 is in close proximity to the optical fibrebundle 60 to ensure that rays reflected from the object plane are notprevented from entering the light guide 2.

[0069] The diffuser 66 overcomes the problem of the optical fibres notacting as a continuous light source. The bundle 60 is made up of a 2Darray of circular fibres separated by the cladding 62 and filing gaps.As a result, the illumination delivered to the edge pixels is notcontinuous and the imagery of those pixels may suffer. The diffuser 66is close to the optical fibre bundle 60, but sufficiently distant toallow homogenising of the discontinuous fibre bundle source (the optimumdistance being in the order of the individual fibre pitch).

[0070] Referring to FIG. 11, a further embodiment of the systemcomprises an optically addressed spatial light modulator (OASLM) 40 inplace of the image screen 18, with a shutter 427 between the OASLM 40and the imaging lens 16. The OASLM is an amorphous siliconphotosensitive layer which modulates voltage across a reflective liquidcrystal layer in response to light. The shutter 42 is electro-opticallyaddressable (a mechanical shutter could also be used) so that only partof the OASLM 40 receives light from the EASLM 6. If the image on theEASLM 6 is changed and the shutter 42 activated to address a differentregion of the OASLM 40 then a highly complex image may be built up onthe OASLM 40 by tiling many separate images from the EASLM 6. It shouldbe noted that the image formed on each segment of the OASLM 40 will bederived from a different one of the virtual EASLM objects, some of whichwill be inverted. Thus, when certain segments of the OASLM 40 are to beaddressed, the object on the EASLM 6 must be inverted relative to thefinal required OAALM image in order to build up the desired pattern onthe OASLM 40. The shutter 42 should be as close as possible to the OASLM40 since the images are not separated until the image plane. Increasedseparation between the shutter 42 and the OASLM 40 will likely result incross-talk between images and/or pixel loss at the OASLM 40.

[0071] In yet a further preferred modification, the shutter 42 isomitted and the OASLM 40 is one which is made sensitive to the incidentlight in specific areas only at any given time, for example bypixelating the electrodes on the OASLM 40. The omission of a shutter 42removes the possibility of cross-talk between images and/or pixel lossat the OASLM 40.

[0072] In yet a further modification (not shown) a scanning mirror islocated between the light source and the light guide. As in FIG. 1, adiffuser is also used, in this case a narrow angle diffuser, between thelight guide and the scanning mirror. In use, illumination of thescanning mirror introduces an angular bias to the light entering thelight guide. By using a narrow angle diffuser, thereby spreading thelight rays over a relatively small angular range, the light can becontrolled so that light can be delivered at any given time to aspecific region of the OASLM (eg. a single image area). Thus, there isno need for a shutter arrangement. In addition, illumination efficiencyis increased since the whole of the OASLM is not illuminated at alltimes.

[0073] From the foregoing, it will be apparent that it may be desirablein certain embodiments to provide a combination of a shutter mechanismwith a selectably sensitisable OASLM and/or a scanning mirror/narrowangle diffuser arrangement (for example when the light is incident onthe image plane over a wider area than required despite the provision ofthe scanning mirror/narrow angle diffuser arrangement).

[0074] It will be readily appreciated by a person skilled in the artthat by cycling through a sequence of different patterns on the EASLM,with time synchronisation of shutter state or OASLM sensitivity, adynamic image can be generated at the OASLM. Such a technique is notunlike the Raster scans used to generate an image on conventionaltelevision screens.

[0075] Referring to FIG. 12, an array of light guides is shown. Sucharrays are advantageous where large pixel counts are required, such asin active tiling systems for use in volumetric imaging or 2D projection.The array comprises a plurality of horizontally arranged mutually spaceddouble sided flat mirror elements 70 and a plurality of spacer elements72 arranged in pairs at spaced intervals therebetween. Each spacerelement 72 of a pair is highly polished on its (flat) side surface 72 afacing the other spacer element of the pair. A light pipe 74 istherefore defined between the mirror elements 70 and each pair of spacerelements 72. A gap is left between adjacent pairs of spacer elements 72so that channels 76 are defined along the length of the array. In use,light is directed down the channels 76 towards the first end of thelight pipes 74, from where the light travels down the light pipes 74towards the object plane as described for the previous embodiments. Itwill be understood that the dimensions of the light pipes 74 aredetermined by the height of the spacer elements 72 and the spacingbetween pairs of spacer elements 72. If necessary, manual adjusters (notshown) can be provided to ensure that the spacing between the spacerelements 72 is precisely as required.

[0076] Referring to FIG. 13, an array is shown which is manufactured ina single piece of metal by casting, the appropriate surfaces alreadybeing reflective (although polishing may still be required). The pieceis cast with rows of channels, the channels alternating between lightpipes 74 and light insertion channels 76. The dimensions of the lightpipes 74 in the array are determined during the manufacturing process.In a slight variation, the array is cast or moulded from a non-metalliccastable or mouldable material (eg. a plastics material) with subsequentapplication of a reflective (eg metallic) coating.

[0077] The use of hollow light guides offers several advantages oversolid glass light guides. Chromatic dispersion in a glass light guidelimits the bandwidth of the light source which can be used (the lightguide acting as a prism). A hollow light guide allows the use ofbroadband sources which do not require filtering. In addition, there isa drop off in transmission through a glass light guide at the blue endof the spectrum. This does not occur with a hollow light guide and alllight is delivered through the light guide. Another significantadvantage relates to the lower refractive index of air compared withglass. The overall imaging performance of the system depends to a largeextent on the flatness of the light guide sides.

[0078] For any given surface deformity, the degree of performancedegradation is directly related to the refractive index of the lightguide—the higher the refractive index, the higher the degradation. Thus,for an allowable level of degradation, the manufacturing tolerances areless stringent for a hollow light guide than for a glass light guide.Furthermore, a lower refractive index allows the length of the pipe tobe reduced, yielding a more compact system, and also one which, due tothe smaller component size, is easier to manufacture.

[0079] The system of the present invention may be used to provide aholographic display. Referring to FIG. 14 a holographic display systemcomprises an image replication system substantially as described withreference to FIG. 11 (only OASLM 40 shown). The optical output of theOASLM 40 is then used to provide a holographic display in a conventionalmanner using a helium neon laser 44 (in different embodiments, otherlasers or alternative light sources such as a notched LED can be used),spinning diffuser 46 and a beam splitter 48 to provide a holographicimage of the object (EASLM 6) which is viewed by a camera 50. Of course,the image can be viewed directly by the human eye.

1. An image replication system comprising: (i) an elongate light guidehaving a polygonal cross section along its length, (ii) a light sourcearranged to direct emitted light, in use, into the light guide at itsfirst end, (iii) a reflective first spatial light modulator opticallycoupled to the second end of the light guide, and (v) focussing meansarranged to receive light reflected through the light guide from thefirst spatial light modulator, said reflected light having exited thelight guide through its first end, and to form multiple images derivedfrom said first spatial light modulator in an image plane.
 2. A systemas claimed in claim 1, wherein the lights source is a source ofnon-polarised light.
 3. A system as claimed in claim 1, wherein thelight source is a source of polarised light, and wherein means forconverting polarised light to non-polarised light are provided in thelight path between the light source and the light guide.
 4. A system asclaimed in any preceding claim, wherein the light source is a source ofcollimated light and wherein a light spreading element is disposed inthe light path between the light source and the light guide.
 5. A systemas claimed in any preceding claim, wherein the light guide is in theform of a solid rod of optically transparent material, preferably glass,having first and second perpendicular polygonal end faces, said endfaces being parallel to the longitudinal axis of the rod.
 6. A system asclaimed in claim 5, wherein the longitudinal edges of the light guideare bevelled.
 7. A system as claimed in claim 6, wherein the bevel depthis about 1% or less of the distance between adjacent longitudinal edges.8. A system as claimed in any one of claims 1 to 4, wherein the lightguide is a hollow pipe, the internal surfaces of the pipe being lightreflective.
 9. A system as claimed in claim 8, wherein the light guideis constructed from discrete reflecting elements.
 10. A system asclaimed in claim 8, wherein the light guide is moulded from glass,plastics or metal.
 11. A system as claimed in any one of claims 8 to 10,wherein the light guide is sealed at its ends by transparent end pieces.12. A system as claimed in claim 11, wherein the light guide isevacuated or gas or liquid filled.
 13. A system as claimed in anypreceding claim, wherein the first spatial light modulator modulates thepolarisation state of incident light.
 14. A system as claimed in claim13, wherein the first spatial light modulator comprises a pixelatedliquid crystal layer on a reflective silicon backing layer, switchedpixels causing modulation of the polarisation state of incident light,unswitched pixels causing no such modulation and/or absorption of lightincident thereon.
 15. A system as claimed in claim 13 or 14, wherein apolariser is provided in the light path between the light guide and thefocussing means, orientated such that only reflected modulated lightpasses therethrough to form an image in the image plane and/or apolariser is provided between the light source and the light guide. 16.A system as claimed in claim 13 or 14, wherein a polariser is providedbetween the light guide and the first spatial light modulator, with afractional wave plate being provided between the polariser and the firstspatial light modulator, and wherein the fractional wave plate is chosensuch that only light passing through the polariser and fractional waveplate and incident on a switched pixel, passes back through the waveplate and polariser upon reflection from the spatial light modulator.17. A system as claimed in any one of claims 1 to 12, wherein the firstspatial light modulator modulates the reflectivity or angular deflectionof incident light.
 18. A system as claimed in any preceding claim,wherein the light guide is provided with an additional light sourcearound its periphery at an end of the light guide which, in use, will beadjacent the first spatial light modulator.
 19. A system as claimed inclaim 18, wherein said additional light source comprises a plurality ofoptical fibres, and a diffusing element is provided between the opticalfibres and the first spatial light modulator.
 20. A system as claimed inclaim 18 or 19, wherein a light steering element is provided to directlight emitted from the additional light source towards edge regions ofthe first light modulator.
 21. A system as claimed in any precedingclaim, wherein an image screen is provided for viewing images formed inthe image plane.
 22. As system as claimed in any one of claims 1 to 20,wherein a photosensitive material is provided in the image plane.
 23. Asystem as claimed in any preceding claim, wherein a second, opticallyaddressable, spatial light modulator is provided in the image plane. 24.A system as claimed in claim 23, wherein the optically addressablespatial light modulator is a ferroelectric liquid crystal lightmodulator on a silicon backing.
 25. A system as claimed in any precedingclaim, wherein means are provided to produce an image from lightreflected from the first spatial light modulator in a predeterminedregion of the image plane.
 26. A system as claimed in claim 25, whereinsaid means comprises means to introduce an angular bias to light fromthe light source prior to entry into the light guide in conjunction witha narrow angle diffuser.
 27. A system as claimed in claim 25 or 26,wherein said means comprises an electro-optical or mechanical shutter.28. A system as claimed in any one of claims 23 to 27, wherein means areprovided to selectably sensitised a predetermined region of the secondspatial light modulator to incident light.
 29. A system as claimed inany preceding claim, wherein means for guiding light from the lightsource to the light guide are provided.
 30. A system as claimed in claim29, wherein said light guiding means comprises a beam splitter betweenthe light source and the light guide and an optical relay between thelight source and the beam splitter.
 31. A system as claimed in claim 30,wherein the optical relay comprises a pair of mutually spaced lenseswith a variable aperture stop therebetween.
 32. A system as claimed inclaim 31, wherein an amplitude mask is provided in the stop of theoptical relay.
 33. A system as claimed in any preceding claim comprisinga plurality of light guides.
 34. A system as claimed in claim 33,wherein said light guides are arranged in an array such that adjacentlight guides are mutually spaced.
 35. An illumination system comprising:(i) a light source and optically coupled thereto, (ii) an elongate lightguide having a polygonal cross section along its length, wherein, inuse, light derived from the light source enters the light guide at isfirst end and is transmitted to its second end, and wherein theintensity of transmitted light at said second end is substantiallyuniform over the area of said second end.
 36. An image replicationsystem or an illumination system substantially as hereinbefore describedwith reference to any one of FIGS. 1 to
 14. 37. An image replicationmethod comprising providing a light source and modulating the lightemitted therefrom using a first spatial light modulator, the methodcharacterised by: directing in a first direction light through anelongate light guide having a polygonal cross section along its length,modulating said light with the first spatial light modulator, reflectingsaid modulated light in a direction opposite to said first directionback through the light guide, and focussing said modulated light in animage plane.
 38. The use of an image replication system in accordancewith any one of claims 1 to 34 or 36 or the illumination system of claim35 or 36 in the image replication method of claim 37.